Light-emitting diode (“LED”) based lighting systems are currently increasing in popularity for a number of reasons. Compared to incandescent lighting (based on filament heating), LED-based lighting systems are much more efficient at conversion of input power to light energy. Compared to fluorescent lighting (based on absorption and reemission of photons generated by a plasma), LED-based lighting systems have longer lifetimes, operate without noticeable flickering and humming, can be dimmed by reducing the operating current thereto, and do not require high voltage electronics.
Efficient removal of heat is important in LED-based lighting systems. Despite its efficiency, heat is generated by an LED during operation, and concentrates in a small volume, potentially increasing the LED's operating temperature significantly. The operating lifetime of an LED is often strongly correlated to its operating temperature, such that a small increase (e.g., a few degrees Celsius) in operating temperature may degrade operating lifetime by hundreds or thousands of hours.
FIG. 1 shows a portion of a prior art LED-based lighting system 10. LEDs 20 and other circuit components 30 mount on a printed circuit board (“PCB”) 40, which in turn mounts on a heat sink 60 (not all LEDs 20 and components 30 are labeled in FIG. 1 for clarity of illustration). PCB 40 includes a metal core 45. A front side 42 of metal core PCB 40 has a dielectric layer 50 and conductors 55 that electrically connect LEDs 20 with circuit components 30 and with external power supplies. The metal core of PCB 40 facilitates heat transfer such that heat generated by LEDs 20 flows through PCB 40 (from front side 42 to a back side, hidden in the perspective of FIG. 1) to heat sink 60. System 10 may also include thermal grease (hidden in the perspective of FIG. 1) between the back side of PCB 40 and heat sink 60 to further facilitate heat transfer.
In a thermal test of system 10, with LEDs 20 being ½ watt LEDs and operated at a given test current, a ΔT (difference in temperature) of 5 to 6 degrees Celsius was measured between metal leads of LEDs 20 and heat sink 60.
Another PCB substrate material that has been utilized for LED-based lighting systems is ceramic material, which can be costly and can introduce manufacturing difficulties, such as low yield when substrates are singulated (separated into single units during fabrication) and difficulty in reworking of mounted components.